This invention relates to ophthalmological instruments and, in particular, to ophthalmoscopes which permit examination, under magnified conditions and proper stereopsis, of the fundus or retina of a patient's eye.
A number of instruments are known for examination of the fundus of the eye. The direct ophthalmoscope is typically a hand-held instrument which illuminates but does not actually magnify the fundus. Because no lenses are employed in a direct ophthalmoscope, other than those which correct for the patient's corneal refraction, the image is erect rather than inverted. An appearance of magnification, known as physiological magnification, occurs in the direct ophthalmoscope because the image of the patient's fundus is larger than expected by the observer. However, since the fundal image is focused by only one of the examining practitioner's own eyes, the view lacks depth.
An indirect ophthalmoscope, on the other hand, uses a magnifying lens, called a condensing or observation lens. This lens is placed in front of the patient's eye and is used to focus a magnified image of fundus between the patient and the observer for stereo viewing. The condensing lens typically is used in conjunction with a binocular device, supported upon the practitioner's head, which serves to reduce the practitioner's interpupilliary distance and provide illumination. The fundal image viewed by the practitioner is inverted and reversed.
Although they produce inverted and reversed images, indirect ophthalmoscopes have gained widespread recognition as diagnostic tools in identifying diseases such as tumors, diabetic retinopathy, retinal detachments and peripheral uveitis. Magnification in the indirect ophthalmoscope is dependent upon the power of the condensing lens. Even though the image viewed by the practitioner with an indirect ophthalmoscope is inverted and reversed, the indirect ophthalmoscope maintains proper stereopsis because the images seen by the left and right eyes of the practitioner are also switched by the instrument. Hence, normal depth perception is reinstated. If the left and right images were not also switched, a condition known as pseudostereopsis would exist in which protuberances appear as cavities, etc.
Despite the advantages of indirect ophthalmoscopes, there exists a need for ophthalmoscopes with better magnification, and with good resolution, particularly to study the fine details around the macula of the eye and to examine tiny lesions. Such instruments desirably provide better magnification without sacrificing proper stereopsis and without sacrificing substantial portion of the field of view so that depth perception is maintained and nearly the entire fundus can be examined.